Well-Log Signature of Fractured Intervals in Carbonate Platform Rocks from the Sergipe Sub-Basin (NE Brazil)

Araujp, Mario C.¹; Matos, Gabriel C.¹; Erthal, Marcelle M.²; Lourenço, Fernanda S.¹; Santos, Maria Rosilene F.³; Lage, Alexandre C.⁴; Almeida, Savio M.⁴; Valverde, Ricardo S.⁴
(1)Research and Development Center, Structural Geology and Geotectonics Division, PETROBRAS S.A., Rio de Janeiro, Brazil. (2) Research and Development Center, Sedimentology and Petrology Division, PETROBRAS S.A., Rio de Janeiro, Brazil. (3) E&P, Sedimentology and Stratigraphy Division, PETROBRAS S.A., Aracaju, Brazil. (4) E&P, Evaluation of Blocks Division, PETROBRAS S.A., Aracaju, Brazil.

Understanding and interpreting timing, location, orientation, and intensity of natural fractures within a geologic structure are commonly important to both exploration and production planning activities. As practiced in the petroleum industry, natural fracture predictions are commonly based on outcrop studies, 3D structural models, seismic attributes, but far less by usage of conventional well-logs. In the Sergipe sub-basin (NE-Brazil), in the area known as Aracaju High, recent oil shows in folded and faulted Albian to Coniacian carbonate rocks stimulated fracture predictions by combination of structural analysis of outcrops and well-logs. This work shows the integration of well-log data, core description, and outcrop studies applied to subsurface prediction of lateral and vertical continuity of intensely fractured intervals.

In the analogue outcrops, distribution and intensity of fractures are influenced mostly by layer thickness, clay content, cementation and structural position. The highest degree of fracturing was found in mudstones interlayered with shales and marls, in which high frequency lamination of thin ductile and brittle layers decreased mechanical thickness and increased fracture intensity. Fracture intensity decreases where carbonate layers are coarse-grained, thicker and poorly laminated. Core analyses show that fractures are vertically distributed in intervals variable in thicknesses that coincide with low values of gamma ray and SP logs. More detail was obtained with density logs, which separated shallow depth intervals of high values from deeper ones with lower values. These signatures are consistent with two different types of fractured reservoirs: shallow low matrix-porosity intervals and deeper intervals with matrix and fracture porosities. Connection of primary porosity by fractures makes the deeper intervals potential candidates to high-permeability reservoirs.

This work confirms that fine-grained interlayered rocks, marked by high frequency laminations of ductile and brittle layers, are more fractured than poorly laminated layers with heterogeneous grain-size. Identification of these fractured intervals with conventional well-logs provides a practical tool for subsurface predictions of lateral and vertical continuity of potentially fractured zones in similar regions of poor seismic resolution.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.